A transmission network which connects bases used in long-distance communications and the like has conventionally been constructed using apparatuses which are based on time division multiplexing (hereinafter abbreviated to TDM (Time Division Multiplexing)) technology such as SONET/SDH (Synchronous Optical NETwork/Synchronous Digital Hierarchy). Frequency synchronization among plural stations connected via the TDM network has been performed through clock recovery from “0” and “1” signals on the SONET/SDH physical layer. On the other hand, recent transmission networks, which are packet communication networks using the IP (Internet Protocol) and the like, are often constructed using devices, such as routers and switches, which use packet technology.
Frequency/time synchronization systems for packet communication networks include the NTP (Network Time Protocol) and IEEE 1588 PTP (Precision Time Protocol). IEEE 1588, which has the potential capability for high-accuracy time synchronization in milliseconds, is planned to be used, for example, for time synchronization and the like among cellular base stations.
FIG. 1 is a diagram illustrating an example of a communication system which uses a packet-based frequency/time synchronization system. The communication system illustrated in FIG. 1 is a cell phone network system and includes a cellular-station side apparatus, a base station (denoted by BS in FIG. 1) and plural rely devices adapted to relay communications between the cellular-station side apparatus and base station. Furthermore, the communication system illustrated in FIG. 1 uses IEEE 1588 as a frequency/time synchronization system, and a master apparatus (hereinafter simply referred to as the master) is installed on the side of the cellular station while slave apparatuses are installed on the side of individual base stations (hereinafter simply referred to as slaves).
As illustrated in FIG. 1, in the packet-based frequency/time synchronization system, frequency/time synchronization is performed as special synchronization messages are exchanged between the master and a slave. The synchronization message contains a sender or receiver time stamp. The slave calculates transfer time between the master and slave based on the time stamp contained in the synchronization message from the master, thereby performs frequency/time synchronization, and provides the synchronized time or clock to the base station and the like.
FIG. 2 is a diagram illustrating a mechanism for calculating an amount of time offset according to IEEE 1588. The time at which the master sends a synchronization message (Sync) is designated as t1 and the time at which the slave receives the synchronization message (Sync) is designated as t2. Also, the time at which the slave sends a synchronization message (Delay_Req) is designated as t3 and the time at which the master receives the synchronization message (Delay_Req) is designated as t4. In this case, the slave calculates the time offset using Eq. (1) below. However, it is assumed that transfer times between the master and slave are equal between both directions. Using the time offset found according to Eq. (1), the slave modifies the time managed by the slave.Time offset={(t2−t1)−(t4−t3)}/2  Eq. (1)    [Patent document 1] Japanese Patent Laid-Open No. 2002-16637
When a packet network which relays communications between the master and slave is made up of rely devices such as switches and routers the rely devices relay user data traffic in addition to synchronization messages. Consequently, user data traffic and synchronization messages may sometimes merge together in a rely device by being inputted through different ports of the rely device and outputted through a same port. When a synchronization message merges with user data traffic, if the user data is already in the process of being read out, the synchronization message waits its turn to be read out until the reading of the user data is completed (see, for example, reference numeral 500 in FIG. 1).
A delay time experienced by the synchronization message as a result of the merge with the user data is difficult to avoid even if the rely device performs QoS (Quality of Service) processing. For example, even if the synchronization message is set at the highest class in terms of QoS, if a low-class packet is already being read on the arrival of the synchronization message, the synchronization message is not read until the reading of the packet is completed. This will result in a delay.
Hereinafter, the merging of a synchronization message and traffic of another packet (e.g., a user packet) on a rely device will be referred to as a conflict. Also, a packet and traffic which conflict with a synchronization message will be referred to as a conflicting packet and conflicting traffic, respectively. Also, a delay in the transfer of a synchronization message caused by a conflict will be referred to as a conflict delay.
FIG. 3 is a diagram for explaining variations in forward delay time of a synchronization message. The timing with which a conflict occurs on a rely device as well as packet length of conflicting traffic vary from case to case, and consequently conflict delay time varies as well. For example, the larger the size of a conflicting packet being read when a synchronization message arrives, the longer the conflict delay time.
Since the conflict delay time varies depending on the situation, the transfer times (forward delay time; t2−t1 and t4−t3 described above; and so forth) of synchronization messages between the master and slave, which contain the conflict delay time and are found by the slave, also vary depending on the situation. For example, the longer the conflict delay time, the longer the forward delay time. Consequently, there is dispersion in the values of time offset found from the transfer times of synchronization messages between the master and slave using Eq. (1). This results in degradation of time synchronization accuracy. Such variation in forward delay is known as PDV (Packet Delay Variation).
FIG. 4 is a diagram illustrating examples of delay distributions of synchronization messages. FIG. 4 illustrates a delay distribution G1 of synchronization messages when conflicting traffic occupies a narrow band and a delay distribution G2 of synchronization messages when conflicting traffic occupies a wide band. As illustrated in FIG. 4, the transfer times of synchronization messages between the master and slave become discrete due to PDV (see G1 and G2). As can be seen from the fact that the delay distribution G2 is more discrete than the delay distribution G1, the degree of discreteness depends on the band of conflicting traffic. In FIG. 4, the delay distribution G2 of synchronization messages when conflicting traffic occupies a wide band contains a smaller number of synchronization messages at and around transfer times at which there are no conflict than the delay distribution G1 of synchronization messages when conflicting traffic occupies a narrow band. That is, with increases in the band of conflicting traffic, a maximum value of delays increases as well, and the number of conflict-free synchronization messages decreases. The conflict-free synchronization messages are synchronization messages which are read without waiting for reading of any other packet of a low priority class in an output queue to be completed, i.e., synchronization messages without a conflict delay.
Even when conflicting traffic occupies a wide band, if a predetermined number of conflict-free synchronization messages can be secured, it is possible to reduce impacts of delay variation during relaying by using the transfer times gained from the conflict-free synchronization messages and thereby improve synchronization accuracy. Therefore, to improve synchronization accuracy on the slave, a predetermined number of conflict-free synchronization messages can be secured without regard to network usage. Since conflict-free synchronization messages are synchronization messages without a conflict delay, to secure a predetermined number of conflict-free synchronization messages, delays of synchronization messages can be reduced.
Incidentally, although time synchronization has predominantly been described above, frequency synchronization, which is performed based on the transfer times of synchronization messages between the master and slave as in the case of time synchronization, encounters problems similar to those of time synchronization.